1. Field of the Invention
This invention is directed to an optical path length modulator for dynamic change of the optical path length of a light beam in optical interferometry, holography and tomography.
2. Description of the Related Art
In optical interferometry, holography and tomography, an always recurring task consists in dynamically changing the optical path length in a beam path. This can be carried out either in the measurement beam or in the reference beam of the interferometer or interferometric arrangement in holography and tomography. A simple example for a path length modulator of this kind is a moving planar reflector mirror which reflects a light beam impinging normal to the mirror surface. Another example of a path length modulator of this kind is described in International PCT Application WO 92/19930, "Method and Apparatus for Optical Imaging and Measurement" (priority date: Apr. 29, 1991; Inventor: Huang, D.; Fujimoto, J. G.; Puliafito, C. A.; Lin, C. P.; Schuman, J. S.). In this reference, a moving roof prism is used as reflector in order to modulate the path length of the reference light beam in an optical tomography device.
Another simple example for a path length modulator of the type mentioned above is a rotating glass cube which changes the optical path length of a light beam passing through it as a function of its azimuth angle .alpha.. This is shown in FIG. 1. In this illustration, the light beam 1 traverses the rotating glass cube 2 which is rotatable or rotates about an axis 3 oriented normal to the drawing plane. In this case, the optical path length increases as the angle .alpha. increases.
While the rotating glass cube is a very simple and practical method and can also work at high speed, it has the disadvantage that the light beam runs through a medium of glass or another transparent material and is therefore subject to wavelength-dependent differences in the index of refraction, that is, dispersion. Particularly in optical short coherence interferometry and tomography, dispersion has the grave disadvantage that the coherence length of the light beam is increased and the resolution of this process is accordingly worsened.
On the other hand, it is also often necessary in the measuring techniques of optical interferometry, holography and tomography to compensate for dispersion existing in the measurement beam through a dispersion of equal magnitude in the reference beam. In this case, a dispersion determined by the optical media of the measured object and the path lengths prevailing therein must be deliberately simulated in the measurement beam. Moreover, in certain cases, the dispersion realized in the reference beam must be dependent on the change in path length; in many cases, it must again be zero. These requirements can also not be met by the rotating cube. This problem is also not solved by the other path length modulators mentioned above.